In the process of photosynthesis, ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate) play crucial roles in converting light energy into chemical energy. These molecules are produced during the light-dependent reactions and are essential for the subsequent light-independent reactions (Calvin cycle).
During the light-dependent reactions, chlorophyll in the thylakoid membranes absorbs light, leading to the photolysis of water molecules. This reaction generates oxygen and releases electrons. The energy from these electrons is used to pump protons into the thylakoid space, creating a proton gradient. As protons flow back into the stroma through ATP synthase, ATP is synthesized from ADP and inorganic phosphate (Pi).
NADP+ is the electron carrier that accepts the energized electrons at the end of the photosystem reactions, along with protons from the stroma to form NADPH. Both ATP and NADPH are then utilized in the Calvin cycle, where carbon dioxide is fixed into organic molecules.
In the Calvin cycle, ATP provides the necessary energy, while NADPH provides reducing power to convert 3-phosphoglycerate into glyceraldehyde-3-phosphate (G3P), a simple sugar. After participating in the cycle, NADP+ and ADP are recycled and return to the light-dependent reactions to be reused. This cycling of ATP, ADP, NADP+, and NADPH is a continuous process, highlighting the interconnectedness of light-dependent and light-independent reactions in photosynthesis.